Operating Systems

In delayed inertia operation the bolt again is not positively
locked and the energy to cycle the system comes from recoil
forces driving the bolt rearward. The difference is that
initial opening of the breech is delayed, typically through
mechanical disadvantage.

This delay can be accomplished in a variety of ways. For
example the WWI-era
Schwarzlose
machine gun
has links between the bolt and receiver which require kinetic
energy to rotate. Due to their arrangement, as the bolt first moves
these links start to turn, absorbing rotational energy.
As the bolt moves back further,
the leverage changes and the energy in the links
is transfered to the bolt.

Another method of delaying blowback is to require
small masses, usually in the bolt, to move at near right angles
relative to the bolt
just after firing. While these masses appear to lock
the bolt to the receiver they actually act as levers
to effectively multiply their mass and the bolt's
against internal recoil forces.

The most successful modern examples of delayed inertia
or delayed blowback arms are the
Heckler & Koch
G3 rifles and MP5 submachine guns and their derivatives.
In the 1950s
HK licensed this design by former Mauser engineer Vorgrimmler
who was then working at the Spanish CETME laboratory.
Vorgrimmler reportedly derived the CETME rifle from his
earlier work on the WWII-era Mauser model 45 assault rifle.

HK describes their operation as roller-locked, delayed-blowback.
In these designs when the bolt is closed,
rollers carried in the bolt are wedged into receiver recesses.
On firing, the rollers must be forced out of the recesses at great
mechanical disadvantage, delaying opening of the bolt,
even with full power 7.62mm NATO (.308 Winchester)
rifle cartridges used in the G3.
The G3 and MP5 fire from a closed bolt and are manufactured
to very high standards resulting in excellent accuracy,
especially as mass produced military arms.
Despite utilitarian stamped and spot-welded sheet-metal construction
and painted finishes,
their simple and smooth operation make them highly reliable
and comfortable to fire.